The present invention relates to the fixation of orthopedic implants, including fasteners to bones and other orthopedic devices and fasteners for effecting such fixation, as well as associated methods.
Orthopedic surgery, whether it be for fracture fixation, joint reconstruction, spine stabilization or fusion, often comprises the fastening of an implant, including an orthopedic plate, to a bone or to another implant or component of a trauma, reconstructive or fusion system. Typically, a screw fastener is used in conjunction with an aperture in an orthopedic implant, and the threads of the screw fastener engage bone or another component of the system as indicated above. In some cases, particularly with respect to certain orthopedic implants that are to be fixed to a bone to treat a fracture, to aid in joint reconstruction or to aid in stabilization or fusion of vertebral bodies, the screw fastener would include not only threads to engage the bony material, but also threads to engage the female threads in a threaded aperture. The engagement of threads between an orthopedic plate, for instance, and the head portion of the screw fastener shank, if any, increases the post-operative strength and stiffness of the plating construct. Such plate-engaging threads help prevent back-out of the screw fastener.
In order to accomplish the goal of fixing orthopedic plates to the bone, orthopedic screws have been constructed with threads designed for engaging bony structures. This enables the screw to be tightly positioned against the plate and bone. In such a design, the aperture of the orthopedic plate in which the screw is received often includes internal threads for coupling the bone screw to the orthopedic implant. Since many screws are designed to specifically have threads which are best adapted for anchoring in bony material, often the engagement between the screw and the orthopedic plate is not optimized.
Prior art designs have attempted to solve this problem by constructing fasteners that have two different threads extending outwardly from a shank. For example, the part of the shank which is designed to be disposed within the bone may include external threads specifically adapted to engage bony material. The part of the shank designed to be disposed within the apertures of the orthopedic plate—i.e., the plate-engaging portion of the shank—includes a second external thread designed to engage the internal threads of the aperture. Often, the second thread has a conical configuration.
A bone screw may also be designed having a conical shank portion. The entire shank of the bone screw may be conical or the conical shank may be limited to only a portion of the shank. For instance, the shank portion that is disposed within an aperture of a bone plate may be conical while the bone engaging portion is not.
Other prior art designs have provided a conical thread from the bone engaging portion and increasing in diameter to the plate engaging portion of the shank of the fastener. The purpose of the conical thread is to self-start the threading of the fastener within the threaded aperture of the orthopedic plate. However, instead, as the conical threads become progressively larger and engage the female threads in the apertures of the orthopedic plate, undesirable cross-threading occurs. This results in a situation where the fastener and the fastener engagement with the plate is weakened. It may also impede the insertion of the screw fastener as the cross-threading may block the passage of the screw. The consequences of such cross-threading, whether full threading was possible or not, is that the screw will be very difficult if not impossible to remove from the plate. Quite often, that aperture of the plate in which the screw fastener is cross-threaded, is destroyed and rendered unavailable for use with another screw fastener.